Coast Trunk Sewer Rehabilitation Project for the Orange County Sanitation District

Transcription

1 Coast Trunk Sewer Rehabilitation Project for the Orange County Sanitation District Paul F. Wilson 1 and Robert J. Warren Faraday Ave, Suite 290, Carlsbad, California, USA 92008; pwilson@pirnie.com Faraday Ave, Suite 290, Carlsbad, California, USA 92008; rwarren@pirnie.com. Abstract: The Coast Trunk Sewer is about 7,300 meters long consisting of 1,350, 1,800, and 2,100 mm RCP PVC lined pipe. The sewer was built in the early 1980 s in Huntington Beach, California to serve a large development that never was constructed and consequently, the flow in the sewer has always been much less than anticipated. Corrosion has occurred to the pipe below the PVC liner that covers only 240 to 270 degrees of the circumference. This paper presents the procedures and technical memoranda that were developed to inspect and assess the condition of the sewer as well as the approach that was followed to evaluate a number of rehabilitation options and ultimately select the alternative most suited for the rehabilitation. The alternatives included 1) replacing the corroded concrete with cementitious grout and maintaining the original shape of the sewer, 2) filling in the bottom of the pipe with concrete, 3) installing concrete weirs inside the pipe to raise the water level, 4) installing a fiber reinforced epoxy liner over the corroded areas, 5) installing a spiral wound pipe liner, 6) extending the PVC liner with PVC bonded to a cementitious grout resurface, 7) sliplining with HDPE and 8) sliplining with centrifugally cast, glass-fiber-reinforced, polymer mortar (CCFRPM) pipe. 1. Introduction The Coast Trunk Sewer is located within the City of Huntington Beach, California, USA. It carries flow from Huntington Beach to the Orange County Sanitation District s Wastewater Treatment Plant No. 2. It consists primarily of 2100 mm (84-inch) and 1350 mm (54-inch) diameter, single-elliptical cage reinforced concrete sanitary sewer pipe (RCP) with a PVC liner. For purposes of this report, the alignment is divided into three segments or zones as shown on Table 1. Table 1: Coast Trunk Sewer Zones Zone Pipe Diameter Manholes PVC Liner Coverage Length 1 1,350 mm ,462 meters 2 1,350 mm 1,800 mm 1,350 mm meters 88 meters 2,118 meters 3 2,100 mm ,587 meters Location Under city streets thru residential and downtown Huntington Beach Under First Street Crossing under Pacific Coast Hwy Under beach parking lot Under beach parking lot and wetlands into treatment plant Water Practice & Technology Vol 3 No 3 IWA Publishing 2008 doi: /WPT

2 The sewer was constructed in the early 1980s to service an area whose demographic projections were not realized. As a result, wastewater flow has been very low and below the termination of the PVC liner installed to protect the pipe from corrosion. Also, the pipe in Zone 3 operates under the influence of the treatment plant. The zone 3 pipe invert is well below the operating level of the plant influent pump station and consequently operates as a full pipe for most of its length. Previous studies had identified corrosion in the sewer but the extent of the damage had not been fully evaluated. The purpose of this project, No Coast Trunk Sewer Rehabilitation, was to quantify and qualify the condition of the Coast Trunk Sewer and provide recommendations for the repair. 2. Method The project was divided into 5 elements that defined the scope of work and set the goals of the project. The elements were: Project Element 1 Preliminary Physical Inspection and Videotaping of the Pipeline Assess the diurnal variation in flow from records and by performing limited depth measurements Conduct physical inspection of the facilities with manned entries into the pipeline to physically measure the amount of corrosion, take close-up digital photographs, and collect limited chemical data (ph of corroded concrete). Project Element 2 Assessment of the Quantity of Solids Deposited in the Pipeline Determine depth and width of sediments for the purpose of calculating estimated quantities of deposited material. Collect samples of the deposits and conduct chemical analyses to determine the characteristics of the deposits. Project Element No. 3 CCTV Inspection Perform a complete CCTV inspection of the entire length of the Coast Trunk Sewer. Project Element 4 Removal of Solids and Final Inspection Remove deposits from Coast Trunk Sewer. Evaluate condition and life expectancy of the RCP conduits and PVC lining material. Project Element No. 5 Preparation of a Pipeline Condition Assessment Report. Preparation of a report that documents the findings and provide recommendations for repair. Prior to any physical inspection that would require manned entry and ventilation, an odor assessment was conducted and an odor control plan was developed based on the findings The odor assessment was conducted by accessing a number of manholes along the alignment and taking measurements at three different times of the day to determine the diurnal variations in the level of sulfides and other odor producing chemicals in the liquid phase and vapor phase inside each manhole. The quantity and location of manholes were selected to develop a full analysis of the odor potential for the entire alignment. Samples were collected by taking grab samples and by setting continuous monitors in select manholes for 24 hour Page 2 of 12

3 fluctuations. Continuous monitors included gas data loggers for the vapor phase and composite samplers for the liquid phase. Significant findings of the assessment were that the level of odor producing compounds in the sewer were low to moderate and that in most cases, the manholes were under negative pressure which indicated a low potential for odor release. Also, there was no health risk from low oxygen levels within the sewer. Data to determine the flow characteristics in the sewer were obtained from a datalogger mounted inside the sewer. The datalogger consisted of a device set on the invert of the sewer at a manhole that measured depth of flow and flow velocity. Measurements were taken every 15 minutes and saved to a digital data storage unit hung inside the manhole. The data was uploaded for analysis on a regular basis. For the purpose of this study, data collected between the dates of November, 2002 thru July, 2003 were analyzed. This period was one of the more recent wet winters on record and provided data during both a wet weather and dry weather period. Analysis of the data was conducted using a popular spreadsheet software program. Initial physical inspection was conducted by manned entry through each manhole supported by a confined space entry crew. The inspector and the confined space entry crew all received special safety training on sewer rescue procedures. The sewer was ventilated using a forced air system to assure safe working conditions. Prior to entry, the air quality at the manhole was tested using a device that measured the oxygen deficiency, concentration of hydrogen sulfide, and level of explosive gases in the sewer. Measurements were taken every 5 minutes and recorded continuously during the manned entry. The individual entering the sewer wore a harness that was tethered to a power winch. He also carried a similar air quality monitor and a two-way radio so that he would know the environmental condition of the sewer and be in constant communication with individuals located above grade. Inspection was conducted at night during periods of low flow and covered a distance of 60 meters upstream and downstream from each manhole, accounting for well over 40% of the total sewer length. Areas of corrosion were photographed and physical measurements were taken. Also, depth and width measurements of sediment deposits were made and samples collected every 15 meters for further chemical analyses. Sediment and liquid samples were transported to a field laboratory unit where immediate chemical analyses could be conducted. Closed Circuit Television (CCTV) inspections were also conducted on the sewer. The inspection was done using a camera mounted on a robotic crawler. Due to the length between manholes, particularly in Zones 2 and 3, some CCTV inspection required the crawler to travel downstream from one manhole and upstream from the next manhole. Also, a procedure needed to be worked out with the treatment plant to permit inspection of the Zone 3 pipe. During low flow hours, the operating level of the plant influent pump station was lowered to dewater the Zone 3 pipe. Also, temporary bulkheads were installed upstream of Zone 3 to further dewater the pipe and assure a complete inspection and thorough inspection. The CCTV inspection included digital video recorded on DVD s and analog video recorded on VHS tapes. A log of the inspection was also made that documented defects and permitted classification and ratings of the pipe conditions according to standards developed by the National Association of Sewer Service Companies (NASSCO). The results of these investigations were used to permit evaluation of the solids found deposited in the sewer and to provide recommendations for repair and rehabilitation of the sewer line. Page 3 of 12

4 3. Results Based on the data collected from the first four project elements, a Pipeline Condition Assessment Report was prepared consisting of a series of technical memoranda that documented the condition of the pipeline and presented conceptual design recommendations of the proposed rehabilitation methods. The following is a list of the technical memoranda and a summary of their findings: 3.1 Odor Assessment and Odor Control Plan Because the odor potential was low and the physical inspection would be conducted during low flow condition between 11:P.M and 5:00A.M., the recommendation was made to locate fans at the siphon on the 2,100 mm portion of the sewer to exhaust the headspace air towards the ocean during the inspection. The beach parking lot closed at 10:00 p.m. so there was limited exposure to the public while the work was in progress. Also, the exhaust was located along a portion of the beach occupied by a power plant which further limited the public exposure. 3.2 Preliminary Physical Inspection The preliminary physical inspection consisted of entry into the sewer at a number of manholes and traversing approximately 60 meters upstream and downstream of each manhole. During the inspection, the condition of the sewer was assessed and digital photographs were taken. Sediment Deposits Results Isolated instances of sediment deposits were noted at manholes and at the downstream end of Zone 2 (where the Coast Trunk Sewer increases in size from a 1,350 mm diameter pipe to a 2,100 mm diameter pipe). The overall length of this portion of the sewer is approximately 4,694 meters. The majority of the deposits (688 cubic meters) were located within the 2,591 meters of sewer in Zone 3 where the pipe is 2,100 mm in diameter. The depth of the deposits ranged from 100 mm to 350 mm. Chemical analyses of the deposits determined that three individual samples out of 23 tested exceeded criteria for toxicity limits for selenium, copper, or barium. Flow monitoring Two flow meters were installed in the sewer during this portion of the study one in the 1,350 mm sewer close to a location currently monitored by the District, and one in the 2,100 mm section of the sewer. The data from the meter in the 1,350 mm sewer closely matched the District s data and demonstrated that the flow velocity averaged about.0.6 m/sec and peaked at about 0.76 m/sec. The depth of flow varied from 200 millimeters to 250 millimeters The average velocity in the 2,100 mm sewer was determined to be about 0.15 m/sc. Preliminary Condition Assessment Piping in Zone 1 was lined for 270 degrees of the pipe s circumference. Piping in Zone 2 had PVC lining for only 240 degrees. The 270 degree lining was adequate to submerge the termination line of the liner and thus protect the portion of the sewer that is bare concrete. Page 4 of 12

5 However, the 240 degree liner was insufficient to submerge the liner s termination edge. Due to the low flowrate in the 1,350 mm and 1,800 mm diameter segments of the Coast Trunk Sewer in Zone 2, flow depths were typically below the edge termination of the PVC liner. Approximately 2,680 meters (38% of total length) of 1,350 mm and 1,800 mm pipe had delaminated PVC liner. Concrete below the PVC liner and underneath the delaminated portions of the PVC liner has been exposed to hydrogen sulfide gas, resulting in concrete degradation and exposed aggregate. Most of the sewer was located in the beach parking lot. The condition of the 2,100 mm (Zone 3) sewer could not be ascertained due to excessive flow depths caused by the operation of treatment plant influent pump station. However, it was possible to measure the depth and width of sediment deposits and collect samples for chemical analyses. Under normal operating conditions, the depth of the sewer is such that it is about 75% full when the influent wetwell is operating at its normal level. While this mode of operation encourages sedimentation inside the sewer, it also protects the pipe because it raises the depth of flow above the termination edge of the liner. 3.3 CCTV Inspection Prior to CCTV inspection, a study was conducted regarding the fate of the solids deposited in the 2,100 mm pipe reach. The estimated cost to remove the solids ranged from $730,000 to $1,580,000 and the benefits were questionable. Due to the influence of the treatment plant, flushing velocities in the sewer cannot be attained and solids will naturally settle in the sewer. Removal of the deposits is a costly procedure with limited disposal options. The deposits do no harm to the sewer and provide a benefit by protecting the bare concrete from exposure to potentially corrosive conditions. Based on the analysis of flow, it was determined that the 2,100 mm sewer could be inspected without bypassing or cleaning if the wetwell at the treatment plant was lowered and upstream flow was held back with plugs during the inspection period. The CCTV inspection of the 1,350 and 1,800 mm sewers confirmed the findings of the preliminary inspection. The inspection of the 2,100 mm reach found one area where corrosion was present. This occurred in a reach where design grade was not maintained during construction. To re-establish invert elevation, the slope of the pipe had been increased to lower the pipe invert by 0.6 meters over a distance of 78 meters which caused the depth of flow to decrease and expose the concrete pipe invert. The pipe walls below the PVC termination edge in a 274 meter reach was found to be as badly corroded as the pipe in the 1,350 mm reach of the sewer. 3.4 Physical Inspection and Condition Assessment As a result of the preliminary physical inspection and the CCTV inspection, a more focused, comprehensive inspection was conducted along the corroded reach of the 1,350 mm and 2100 mm sewer reaches. The inspection involved manned entry with a high pressure sprayer to wash the corroded concrete to remove any soft concrete that might be present to determine the extent of exposure of reinforcing steel. Due to the location of the corroded section of the 2,100 mm pipe, access with a high pressure sprayer was not possible, but a close visual inspection was conducted that defined the scope of the corrosion. The results of the inspections are summarized in Table 2 as follows: Page 5 of 12

6 Table 2: Summary of Inspections of the Coast Trunk Sewer PARAMETER ZONE 1 ZONE 2 ZONE 3 Condition Satisfactory Damaged Isolated Damage PVC Liner Delamination Maximum Depth of Corroded Concrete Minimum ph of Concrete Sediment Level None 1 st T-Lock rib detached from pipe wall for over 2,100 meters No corrosion found 50 mm 90 mm Not measured Heavy in curved section of sewer None found 1 st T-Lock rib detached from pipe wall on a 274 meter section Heavy Exposed Steel Joint Yes, at several None Spigot locations None ExposedJoint Yes, at several None None Gasket locations NASSCO Rating Structural O&M Structural O&M Structural O&M Severity Overall Rating NASSCO Quick Grade NASSCO Grade Score Preliminary Recommendations Re-inspect in 10 years QQQ 524J Repair delaminated PVC liner and concrete in 0 to 2 years Repair delaminated PVC liner and concrete in 0 to 2 years The integrity of the PVC liner was determined to be excellent except where the corrosion extended about 100 mm up under the termination edge. The maximum depth of corrosion in the 1,350 mm sewer was measured at 75 mm, which is half of the 150 mm pipe wall. The maximum circumferential length was 275 mm. The maximum depth of corrosion in the 2,100 mm sewer was 87 mm and the maximum circumferential length was measured at 400 mm. Due to the elliptical configuration of the reinforcing steel, the depth of coverage at the point of maximum corrosion exceeded 75 mm and none of the reinforcing steel was exposed. In a number of locations, the depth of corrosion was severe enough to expose and corrode the metal bell and spigot connections at the end of each fabricated length of pipe. 3.5 Rehabilitation Options and Recommendations Eight alternatives for rehabilitation were evaluated against a series of criteria to determine the best alternative for the Coast Trunk Sewer. The alternatives included 1) replacing the corroded concrete with cementitious grout and maintaining the original shape of the sewer, 2) filling in the bottom of the pipe with concrete, 3) installing concrete weirs inside the pipe to raise the water level, 4) installing a fiber reinforced epoxy liner over the corroded areas, 5) installing a spiral wound pipe liner, 6) extending the PVC liner with PVC bonded to a cementitious grout resurface, 7) sliplining with HDPE and 8) sliplining with centrifugally cast, glass-fiber-reinforced, polymer mortar (CCFRPM) pipe. Page 6 of 12

7 The criteria for evaluation of the alternatives included estimated construction costs, structural value, flow carrying capacity, odor and corrosion control, constructability and maintenance requirements. There also was an issue regarding the location of the sewer. The portion of the sewer that required repair was located under a parking lot utilized by the community for access to the Pacific Ocean. The community was very sensitive to any limitations that the rehabilitation activities may have had on their ability to use the parking lot as well concerns about the potential release of sewage and odors during construction, particularly from exposed bypass pumping equipment. A summary of the results for the feasible alternatives are listed in Table 3 below. The cementitious grout rehabilitation, by itself, was not considered a feasible alternative because it did not resolve the root cause of the corrosion. However, it is a necessary step for preparation of the existing surface for the epoxy liner as well as extending the PVC liner. Using concrete to fill in the pipe invert would raise the water surface above the bottom of the liner, but reduced the pipe carrying capacity to unacceptable levels. The same applied to using concrete weirs to raise the water level. This alternative also would encourage sedimentation and increased odor production. The epoxy liner was unacceptable because there is no apparent method available to transition from the epoxy to the existing PVC liner without exposing the underlying concrete to corrosion..utilizing PVC repair strips to cover the corroded areas was also unacceptable due to complicated preparation procedures, safety concerns while working in confined spaces, requiring ventilation, and public exposure to increased odors during construction and bypassing operations in the parking lot. The three remaining alternatives; spiral wound pipe liner, sliplined HDPE, and CCFRPM liner addressed the corroded area by lining the entire circumference of the pipe. Sliplined HDPE was eliminated because of cost and impacts to the community due to the large amount of lay down area required. The construction footprint for the CCFRPM liner was the smallest of all alternatives because the liner could be installed from one location for the entire 2,680 meter reach of the 1,350 mm pipe. However, it would also require replacement of all the manholes and the total cost was prohibitive. Page 7 of 12

8 Table 3 : Rehabilitation Alternative Matrix Rehabilitation Alternative Construction Cost Structural Integrity Feasible Alternatives for Repair and Rehabilitation Cementitious Grout Repair Concrete Weir Concrete Fill Fiber Reinforced Epoxy PVC Liner Repair Spiral Wound Pipe Liner HDPE Slipline CCFRPM Slipline $490/m 2 $2.0 million $577/ m 2 $2.8 million $1,120/ m 2 $3.8 million $1,155/ m 2 $5.3 million $1,400/ m 2 $6.9 million $1,290/ m 2 $5.4 million $1,930/ m 2 $8.5 million $2140/ m 2 $9.4 million No Impact No impact No impact Improved No impact Hydraulic Capacity No reduction 67% reduction 41% reduction No reduction No impact Odor Potential/ Corrosion Control No change Control decreased Potential Increased Potential Increased No Change No change No Change Improved 12% reduction Improved Improved 20% reduction 18% reduction No Change No Change Constructability Continuous bypass, extensive prep work, manned entry, traffic control Continuous bypass, minor prep work, manned entry, traffic control Continuous bypass, minor prep work, manned entry, traffic control Continuous bypass, minor prep work, manned entry, traffic control Continuous bypass, extensive prep work, manned entry, traffic control No bypass, minor prep work, reduced manned entry, minor traffic control No bypass, minor prep work, minimal manned entry, traffic control No bypass, minor prep work, minimal manned entry, traffic control Maintenance Requirements Increase Increase Increase Increase No change Decrease Decrease Decrease Page 8 of 12

9 4. Summary The spiral wound pipe liner ultimately was selected for the repair because it provided a viable solution with a small construction footprint and minimal impact to the surrounding community. The project had the distinct advantage of extremely low flow which permits this technology to be used without the need for bypassing. Also, since the liner is grouted in place, the need to prepare the corroded surface prior to applying cementitious grout or any of the partial liners was negated. Finally, the exposure of the installation crew to confined space issues and the need to ventilate was minimized by utilizing spiral wound liner technology. For most installation methods, the equipment used to install the liner is fixed in one location from where approximately 450 meters can be installed. Table 4 is a summary of the recommendations and future action for operation of the Coast Trunk Sewer. Table 4: Summary of Recommendations Issue Recommendation Action Rehabilitation Construction Maintenance Monitoring Zone 1 No rehab work required Zone 2 Spiral Wound Pipe Liner Zone 3 Spiral Wound Pipe Liner in 274 meters of pipe Construct rehab following design plans for sequence of work Limit bidding to experienced contractors Establish Maintenance and Monitoring Plan for entire Coast Trunk Sewer Leave sediments in 2,100 mm sewer Zone 1 Re-inspect in 10 years Zone 2 Inspect manholes and clean annually Zone 3 Inspect manholes annually Design Phase shall establish final scope of rehab and maintenance impacts Work with the City of Huntington Beach to establish and coordinate bypassing options Include experience clause in construction bid package Design Phase shall establish final impacts to maintenance activities Maintenance and Monitoring Plan developed during design or construction phase Create and implement sustainable Maintenance and Monitoring Program based on post-rehab construction conditions Odor Control Maintain current odor control practices No further action required. 5. Conclusion A thorough inspection of sewers in municipal settings demands attention to impacts on the public that can occur during the inspection. The inspection of the Coast Trunk Sewer has demonstrated that lines made of RCP must be protected for 360 degrees of their circumference to assure that corrosion will not occur. When corrosion does occur there are many alternatives that are available for rehabilitation which should be evaluated based not only on cost, but also on their impact to the surrounding community and on the requirements of the owner. Page 9 of 12

10 REFERENCES 1. California Code of Regulations Title 22 Division 4.5 ( ) governs treatment or disposal of hazardous wastes and sets limits used to provide criteria for identification and classification of hazardous wastes. 2. The National Association of Sewer Service Companies (NASSCO). Page 10 of 12